As an image forming apparatus of an electrophotographic type, a printer having a photosensitive member on which a developer image is formed has been known.
For example, a laser printer that has four photosensitive drums, four transfer rollers and a belt has been known (for example, refer to JP-A-2007-41348).
In the laser printer, the four photosensitive drums correspond to black, yellow, magenta and cyan, respectively. Also, the four transfer rollers are positioned just below the four photosensitive drums, respectively. The conveyance belt conveys a print sheet so that it sequentially passes between all the photosensitive drums and all the transfer rollers.
In the laser printer, while the print sheet is conveyed, an adhering substance (for example, paper dust, developer and the like) that is adhered on a surface of the conveyance belt is accommodated in a casing of a belt cleaning device via a first surface roller and a second surface roller by applying a bias to the first surface roller.
In the laser printer disclosed in JP-A-2007-41348, the bias is applied to the first surface roller. Therefore, when the laser printer is used for a long time, so-called energization deterioration is caused in which an electric resistance of the first surface roller is varied by current flowing to the first surface roller.
Here, when constant current control of controlling the current flowing to the first surface roller to be constant is performed, if the electric resistance of the first surface roller is varied, it is necessary to vary a voltage to be applied to the first surface roller so that the current flowing to the first surface roller becomes constant.
Therefore, in order to realize the constant current control to the first surface roller which is energization-deteriorated, if a power supply which allows variation in the voltage to be applied to the first surface roller and is capable of greatly varying the voltage is provided, the cost of the image forming apparatus increases.
Also, in order to suppress the cost of the image forming apparatus from increasing, if the belt cleaning device is replaced when the electric resistance of the first surface roller does not exceed a predetermined value, it is difficult to prolong a replacement period of the belt cleaning device, so that it is difficult to use the belt cleaning device for a long time.